AD8232 ADC recommendation

Could I get some advice on selecting ADC for the AD8232 device please. Would the AD7190 be a good choice for AD8232? We have been trying to use the ADuCM360 built in ADC by following the sample schematics from the AD8232 data sheet and would like to explore alternatives as well.

The recommendation on the data sheet is based on the idea of saving power and reducing area by increasing integration. However, there are numerous options depending on your application. Are you trying to get very high resolution? consume the lowest possible power? Are you interested in the shape of the waveform or simply in counting heart beats? I can probably give you a few pointers, but in the end, you may want to post your question in the precision converter community for a better recomendation:

Thank you very much for your reply. I will post the question to the precision adc community, but your expert advice and pointers would be definitely useful as well, and your help is very much appreciated. I am interested in the waveform so not just implementing a heart rate monitor and low power consumption would be a plus.

To narrow it down, I would consider the noise from the AD8232 first. The peak to peak noise of the instrumentation amplifier, referred to input (RTI) is 14uVpp in a frequency band of 0.1Hz to 40Hz. Because the gain is 100, that is equivalent to about 1.4mVpp at the output. The uncommited opamp has its own noise as well, but because you would use it after the instrumentation amplifier, its noise contribution is negligible. However, if you add gain of say, 4 (for a total of 400) with the opamp (and filter at 40Hz), the total noise at the input of the ADC would be 5.6mVpp. If you were to digitize this signal with an ideal ADC, and because the maximum swing of the amplifier would be roughly 3V, you'll get an ENOB (or noise free bits) of just over 9 (3/5.6mV=536 steps vs 2^9=512). This means that a 12-bit ADC is a good start for a low-cost, low power ADC.

Because the uncommited opamp can be used as a filter to limit the noise and to prevent aliasing, you don't need a great deal of oversampling. If your corner frequency is 40Hz, a second-order filter (like the Sallen-Key proposed in the data sheet) will have a rejection of 80dB when it reaches 4kHz. This means that sampling at 4kHz with a 12-bit ADC will most likely not introduce any aliasing into your acquisition and would be the minimum sampling rate you would want to use to reconstruct the signal.

With this information, you know the minimum you need to get a decent signal. But I have assumed that a gain of 400 is enough for the signal amplitude that you have at the input, but this may vary with electrode placement, electrode type, etc. And that's where you need to consider if you need additional signal resolution. For example, if you expect variation in your signal amplitude, you may want to use the minimum gain of 100 to avoid saturation when the signal is strong. This also means that the noise will be 4 times smaller, and that the same signal we discussed before (with a gain of 400) will also be 4 times smaller. Under these conditions, you may want to use a higher resolution ADC to get a better resolution from smaller signals. For this example, I have reduced the gain by a factor of 4, so, just based on the noise, you would need 2 more bits for the same ENOB, which could push you to a 14-bit ADC.

Both the AD7190 and the ADC(s) in the ADuCM360/1, are sigma-delta that can achieve very high resolution. One of the things you get from a sigma-delta ADC is the ability to change the update rate (or reading rate) with the same input sample rate. In other words, if you increase your oversampling ratio, you get more resolution. If you look at the resolution tables in their respective data sheets you'll see that for 4kHz update rate, the AD7190 yields better resolution than the M360/1, but probably more than you'll need (because you can't get less than a gain of 100 with AD8232). So you'll be gettting less integration, consuming more power, and possibly paying a higher price for no real benefit, since the AD8232 will be the limiting factor. the ADuCM360/1, in my opinion, is a better deal for the application (and lower power too).

Here's another idea: if you'd like to use a stand-alone ADC and save power/area/cost, you could take a look at AD7091. This is a very low-power 12-bit SAR ADC.In a general sense, with a SAR, you don't get less resolution when you get lower update rates (which are equivalent to the sampling rates), but you get lower power consumption. I didn't include it in the data sheet because it wasn't available at the time AD8232 released, but I think it would be a good match for the application.

The quality and level of support you have been providing is absolutely amazing.

The ADuCM360 is great option, but we have an MCU already in the system to handle wireless communication and therefore I thought a stand alone ADC would save power, money and space - though the last probably not necessarily true as the ADuCM360 has a small footprint. On that note, the package of the ADuCM360 seems introduces some issues as we can not find a programmer socket for the 48-lead LFCSP package, and therefore this issue lead us as well to consider using a stand alone ADC and not to use the ADuCM360, which otherwise would be a good start as your datasheet at least give us some excellent starting point using the ADuCM360.

We will try to experiment with the AD7091 straight away and we will order some development boards. I can't see explanation about chained configuration in its data sheet, as I thought we would need two AD7091 with the AD8232. However the data sheet indicates that we would need to use a multiplexer to acquire multiple signals, which I guess is the case using the AD8232. Because of the timing of the multiplexer, I am not sure whether would that work with the AD8232 when we are interested int the shape of waveform? Or maybe I just completely misunderstand how to use the AD7091 with the AD8232. Probably it is just easier to make some progress using the ADuCM360 to get a shaped waveform and visualize an ECG signal. I am just think loudly about what to do to make some progress, but any of your input would be very useful :-))

Again, thank you so much for you absolutely first class support and your help is highly appreciated.

I appreciate your compliments; I hope you and the EngineerZone community benefit from this discussion.

I have to say that it took me a little while to realize why you think you'd need two ADC inputs to read the output of AD8232! Anyway, I'm glad you brought that up, because it may be confusing for other people too, since I didn't make this explicit on the data sheet...

The output signal from the AD8232 is riding on top of a half-supply voltage (as shown in figure 68 in the data sheet) because the circuit is operating from a single-supply. If you have a differential input available (such as in the ADuCM360/1), you'll get your signal centered around "zero". However, if you only have a single-ended ADC input, such as with AD7091, you can still get the signal from the output of the uncommited opamp (or even the inamp if you wish) referenced to ground. The difference is that the signal will be sitting on top of that half-supply offset.

What that means is that with a single-ended ADC, the DC level of the signal can change by as much as the half-supply voltage does, which is determined by the resistor divider on REFIN. What I mean by "DC level" is the voltage present at the output when the inputs of the inamp are shorted together (no signal present). Say that your reference level is set by the ADC reference at 2.5V, you have a +/-0.1% tolerance on your divider, and you expect mid-reference to be your signal's reference level (1.25V). In this case, you'll see as much as +/-1.25mV variation (or 4LSBs with a 12-bit ADC). If you used the power supply to generate such level, (different from the ADC's reference) you'll also see half as much of the supply variation at your output (which can change upon loading conditions). And I have not considered any temperature effects yet. You don't have any of these issues with a true differential input, and the only offset and drift you'll ever see is from the amplifiers (which is usually smaller). Another subtle difference at the system level is how the signal is represented in the digital domain: as a 2's complement (for a differential input) or unsigned binary (for a single-ended input).However, with a 12-bit ADC, you may not have enough resolution for all of this to be a problem. And the AD7091 derives its reference from the supply, so any supply variations are ratiometric and won't be seen by the ADC.

At this point, the chained configuration should not be an issue any longer. But I don't think it is possible to do that with the SPI implementation used on AD7091.

Your latest reply just like the previous was extremely useful. Thank your explanations we have already lot better understanding and cleaner ideas how to deal with the AD8232 device.

We will go forward on a dual route with the AD8232 development - using the ADuCM360 in one experiment and a stand alone ADC on the other system. Once all components in place and we start to see the signals I am sure the majority of problems will start for us and we will have to ask your expert opinion again.

I am really grateful for your help, thank you very much for sharing your knowledge with us.

Just one quick additional note. The single ADC channel version of the ADuCM360 just released: the ADuCM361. If you don't need the extra channels, you may be able to save some money (although you can prototype with the M360). If you need specific support with the M360 or M361, you can post your questions on the Cortex M3 community:

Thanks for the info. I am sure we will end up at the Cortex 3 community forum :-)) but you have provided us with an extremely useful starting points so we should be able to make progress terms of acquiring the signals.